Abstract
A triad of interacting group (TyrOHHis\documentclass{article}\pagestyle{empty}\begin{document}$ \underline\ominus$ \end{document}O~2~C) in angiotensin II (ANG II) has been postulated to create the tyrosinate anion pharmacophore (tyanophore) responsible for receptor activation/triggering (Biochim. Biophys. Acta 1991, 1065, 21). In the present study we investigated the effects on bioactivity of substituting the Tyr^4^ residue in [Sar^1^]ANG II with other anionic or electronegative amino acids, and with a number of aromatic amino acids lacking a hydroxyl group. [Sar^1^ Nva(δ‐OH)^4^]ANG II, [Sar^1^ Nva(δ‐OCH~3~)^4^]ANG II, [Sar^1^ Met^4^]ANG II, [Sar^1^ Gln^4^]ANG II, [Sar^1^ Glu^4^]ANG II and [Sar^1^ DL‐Alg^4^]ANG II had agonist activities in the rat isolated uterus assay of 4, 3, 19, 10, > 0.1 and > 0.1%, respectively, of that of ANG II. [Sar^1^ Nal^4^]ANG II, [Sar^1^ Pal^4^]ANG II, [Sar^1^ DL‐Phg(4′‐F)^4^]ANG II, [Sar^1^ Phe(4′‐F)^4^]ANG II, [Sar^1^ Phe(F~5~)^4^]ANG II and [Sar^1^ His^4^]ANG II had agonist activities of 4.5, 7, < 0.1, 0.2, 1 and 0.6%, respectively. All peptides investigated were devoid of measurable antagonist activity except [Sar^1^] Phe(4′‐F)^4^ ANG II (pA~2~ = 7.7). These findings illustrate that anionic or electronegative aliphatic side chains replacing tyrosinate at position 4 can partially activate the angiotension receptor. For ANG II analogues containing an aromatic amino acid other than Tyr at position 4, ligand binding and agonist activity are not dependent on the electronegativity or dipole moment of the aromatic ring, or on the ability of the 4′ ring substituent to accept a proton. Modelling based on ab initio calculations of aromatic ring multipoles illustrate that the apparent binding affinity (PA~2~) of ANG II analogues is associated with a perpendicular electrostatic interaction of the position 4 aromatic ring with a receptor‐based group. In addition, intramolecular interactions providing for the conformation of the ligand as it approaches its receptor appear to have a role in determining agonist vs antagonist activity.